var num=0;
function jsq(num) {
         //获取当前输入
         document.getElementById('screenName').value += document.getElementById(num).value;
       }
       function eva() {
         //计算输入结果
         document.getElementById("screenName").value = eval(document.getElementById("screenName").value);
       }
	   function qufan(){
		   //取反
		   document.getElementById("screenName").value=-(document.getElementById("screenName").value);
	   }
       function clearNum() {
         //清屏
		 document.getElementById("screenName").value=null;
       }
      
       function onLoad(){
         //加载完毕后光标自动对应到输入框
         document.getElementById("screenName").focus();
}